Assessing the Subseasonal Predictability of PV Streamer Activity compared to Tropical Cyclone activity using the Navy Earth System Model

Potential vorticity streamers (PVSs) are elongated filaments of high PV air that can serve as channels for tropical-extratropical air mass exchange. PVSs often originate from anticyclonic Rossby wave breaking (AWB), where upstream low PV air is advected poleward over downstream high PV air in the upper troposphere. This flow evolution results in an elongated positively tilted upper-tropospheric trough (i.e., PVS) downstream of the AWB axis. In the warm season, subtropical PVSs modify the tropospheric environment by enhancing vertical wind shear (VWS) and moisture anomalies in their vicinity. These environmental changes play an important role in enhancing or suppressing nearby tropical cyclone (TC) activity in oceanic basins. Given the importance that PVSs play in modifying their local environment in the subtropics, predicting the correct magnitude and location of PVS activity on subseasonal timescales may also be important in determining corridors where TC activity is impacted.

This study uses the Navy’s Earth System Model (NESM) to investigate the subseasonal predictability of PVSs in the northern hemisphere during the warm season (June–November). In the NESM, 45-day reforecasts from 1999-2015 are used to determine predictability of PVSs compared to the verifying analysis. PVSs are identified in these reforecasts on the 350-K isentropic surface bounded by the 2-PVU contour, where PVSs are defined as the high PV trough axis that occurs downstream of the AWB axis. PVS events are also categorized by their area and intensity in order to show regions where repeated PVS occurrence is prevalent in NESM, and also to compute an activity metric incorporating PVS size and intensity for a reforecast period. Skill scores and mean state biases are used to determine the predictability of PVS activity between 1–6 week lead times. Finally, these results are compared to corresponding indices that determine TC activity to see if increased/decreased skill in predicting PVS activity is also related to similar skill changes of TC activity in NESM reforecasts.